How big of a port to make for (4) 10" CVR

how big of a port would i need to make for a box for 4 10" kicker cvr subs. The box i measured out is: Height-16", Width-30", Depth-32". That gives me 7.3 cubic feet and 1.8 cubic feet a speaker. I want to have one big port going out of the bottom but i need to know how long and how big to build it. if anyone can help, please do.